(1) Field of the Invention
This invention relates to a catalytic isomerization process of dimethylnaphthalene. Particularly, this invention relates to a catalytic isomerization process suitable for producing the 2,6-isomer from a mixture of dimethylnaphthalene isomers.
(2) Description of the Prior Art
Dimethylnaphthalene consists of 10 isomers, which can be classified into the following four groups. Their isomerization can be readily carried out within their respective groups but is difficult to achieve between their respective groups.
(1) 1,5-, 1,6- and 2,6-isomers; PA1 (2) 1,8-, 1,7- and 2,7-isomers; PA1 (3) 1,4-, 1,3- and 2,3-isomers; and PA1 (4) 1,2-isomer.
A principal end use of dimethylnaphthalene is the production of naphthalenedicarboxylic acid. For this application, the 2,6-isomer has found particularly preferred utility. On the other hand, the dimethlynaphthalene fraction separated from the recycled oil of the FCC process, whose fraction is the one of industrial supply sources of dimethylnaphthalene, or from coal tar fraction, is a mixture of ten isomers. In order to obtain the 2,6-isomer in a large volume, it is thus necessary to isomerize the other isomers into the 2,6-isomer. Many examples have been known with respect to isomerization of the above isomer within their respective groups, but isomerization between their respective groups has scarecely been known. For example, it has been proposed to perform the isomerization of dimethylnaphthalene by using mordenite, which is a sort of zeolite, as a catalyst (see, Japanese Patent Publication No. 47,020/1980). However, the above catalyst is unsuitable for isomerization between the groups.